1. Field of the Invention
The present invention relates to an integrator adaptable for a current detector, such as a sensor with a current mode output, and relates to an image read device using the integrator.
2. Discussion of the Related Art
Conventionally, an image read device used as an image reader for reading an image on a document original in a facsimile, for example, may be expressed by a simple equivalent circuit shown in FIG. 8. As shown in FIG. 8, light containing image information of the original illuminates a photocell 51 consisting of a constant current source 81 operating according to an amount of light, and a photo diode 82, both being coupled in parallel to each other. The photocell 51 discharges an amount of charge which depends on an amount of incident light. A voltage produced across the photodiode capacitance 82 is picked up through a buffer 83. The voltages thus picked up are multiplexed into time-sequential data of the image signal.
The image read device suffers from such a problem that the wires from the photocells 51 to the buffers 83 are capacitively coupled, providing inexact image data.
To solve the problem, an approach as shown in FIG. 5 has been made. As shown in FIG. 5, one end of each photocell 51 is connected to a common wire 53. The current flowing into the common wire is converted into a voltage by an integrator 1. This approach successfully can reduce the capacitive coupling.
However, a basic integrator 200 as shown in FIG. 9 used in the above approach involves the following problem. In a negative feedback portion of the integrator 200, an analog switch 202 constructed with a CMOS element is provided for resetting an integrating capacitor 201. The quantity of the charge caused by a drive pulse from the analog switch 202 is approximately 10 pC. The quantity of charge leaked from the photodiode forming a sensor 100 coupled with the integrator 200 is 0.1 pC per pixel when it is used under general conditions. Accordingly, most of the output signal of the integrator 200 consists almost all of the noise from the analog switch 202. Additionally, in a state where the analog switch 202 for resetting the integrating capacitor 201 is closed, an operational amplifier 203 tends to oscillate. To avoid this, a response speed of the operational amplifier 203 must be considerably decreased.
The leak charge from the analog switch 202 may be reduced by reducing an area of the MOS element. However, if the area is reduced, on-resistance of the MOS element is increased. With the increased on-resistance, a long time is taken for resetting the integrating capacitor 201. This acts as a negative factor in the high speed operation of the device. Providing the analog switch 201 as an active element in the first stage of the circuit possibly amplifies the noise generated in the switch itself.
In order to solve the above problems, a circuit as shown in FIG. 10 has been developed and practically used. In the circuit, the integrator 200 follows a current-voltage converter 300 including an operational amplifier 301 and a negative feedback a resistor 302. The circuit converts an input current (sensor current) based on a minute quantity of charge discharged from the sensor 100, into a voltage by the current-voltage converter 300. In the circuit thus constructed, the current-voltage converter 300 has theoretically a low output impedance. If a value of a resistor 400 inserted between the current-voltage converter 300 and the integrator 200 is properly varied, the current many times as large as the current from the sensor 100 may be fed to the succeeding integrator 200. As a consequence, the leak charge from the logic of the analog switch 202 can be reduced relatively.
The preceding current-voltage converter 300 uses the resistor 302 for the negative feedback. If a capacitive element, such as the sensor 100 containing a sensor output capacitor 101, is directly coupled with the current-voltage converter 300, the phase of the voltage fed back delays, so that the current-voltage converter will oscillate. To suppress the oscillation, a resistor 500 must be inserted between the sensor 100 and the current-voltage converter 300. Accordingly, a read speed of the circuit is limited by a CR low-pass filter consisting of the sensor output capacitor 101 and the resistor 500.
The response speed of the current-voltage converter 300, when operating at a high speed, is determined by the through-rate of the operational amplifier 301. The output voltage of the amplifier 301 fails to follow the input current (sensor current).
A problem exists, which is common in both the circuits of FIGS. 9 and 10. The problem arises from the fact that the on/off speed of the analog switch 201 is slow, that is, several tens to several hundreds nsec. The slow switching speed consumes excessive time in a high speed operation, and possibly introduces an error.